1. Field of the Invention
The present invention relates to an optical semiconductor apparatus for solving the problem of polarization dependency in the field of optical frequency division multiplexing (FDM) communication, for example. The problem of polarization dependency arises when a recieving sensitivity fluctuates due to changes in the polarization state of signal light on the receiver side. The present invention also relates to an optical communication system and an apparatus using the above-mentioned optical semiconductor apparatus.
2. Related Background Art
In recent years, increased transmission capacity in the field of optical communications has become desirable, and the development of optical FDM communication, in which signals at a plurality of optical frequencies are multiplexed in a single optical fiber, has been advanced.
There are two kinds of optical FDM communication methods, which are classified by the type of light signal used in the receiving technique. One method is a coherent optical communication in which a beat signal is produced between signal light and light from a local oscillator to obtain an intermediate-frequency output, and that output is detected. The other method is one in which only light at a desired wavelength or optical frequency is selected by a tunable filter, and the thus-selected light is detected. The latter method, known as an optical-frequency changeable filtering method, will be described.
The tunable filter may comprise one of a Max-Zehnder type, a fiber Fabry-Perot type and an acousto-optic (AO) type, which have been respectively developed, but each has drawbacks.
In the Max-Zehnder filter type and the fiber Fabry-Perot filter type, the transmission bandwidth can be relatively freely designed and a width of several .ANG. can be obtained, so that the frequency multiplicity of optical FDM communication can be increased. Further, there is a great advantage in that the polarization state of signal light does not adversely affect the quality of signal receiving. An example of a Max-Zehnder type filter is disclosed in K. Oda et al. "Channel Selection Characteristics of Optical FDM Filter", OCS 89-65, 1989. An example of a fiber Fabry-Perot type filter is disclosed in I. P. Kaminow et al. "FDMA-FSK Star Network with a Tunable Optical Filter Demultiplexer", IEEE J. Lightwave Technol., vol. 6, No. 9, p. 1406, September, 1988. Those filter types, however, have the disadvantages that considerable light loss exists and that downsizing of a receiver device is difficult because the integration of a semiconductor photodetector and the filter is impossible.
In the AO modulator filter type, the receiving control is easy since the transmission bandwidth is large, e.g., several tens of .ANG., but the multiplicity of transmitted wavelengths cannot be increased. An example of an AO modulator type filter is disclosed in N. Shimosaka et al. "A photonic wavelength division/time division hybrid multiplexed network using accoustic tunable wavelength filters for a broadcasting studio application", OCS 91-83, 1991. This filter type, however, has the drawbacks that light loss exists, that the integration with a semiconductor photodetector is impossible and that polarization control of signal light is necessary because the polarization state of signal light adversely affects the quality of signal receiving.
On the other hand, in a semiconductor filter type, e.g., a distributed feedback (DFB) filter provided with a diffraction grating formed in a light guide layer for single longitudinal mode operation, the transmission bandwidth can be narrowed (e.g., by several .ANG.), the optical amplification function (approx. 20 dB) exists, the multiplicity of transmitted wavelengths can be increased and the minimum receiving sensitivity can be improved (i.e., the minimum receiving intensity can be reduced). An example of a semiconductor type filter is disclosed in T. Numai et al. "Semiconductor Tunable Wavelength Filter", OQE 88-65, 1988. Further, this type of filter can be formed with the same material as a semiconductor photodetector, so that integration and downsizing are feasible.
From the foregoing, the suitability of a semiconductor DFB type optical filter for optical FDM communications is clear.
The recent situation of signal modulation systems is as follows. At present, the most popular modulation system for transmission systems using optical filters is a digital amplitude modulation system or amplitude shift keying (ASK). There are two methods for implementing this type of modulation. One is a method in which current injected into a laser diode (LD) is directly modulated, and the other is a method in which an external intensity modulator is employed. The former method is not suitable for a high-density wavelength multiplicity since wavelength chirping (e.g., of several .ANG.) occurs in the LD. The latter method has the drawback that the use of the external modulator entails light loss therein. For the foregoing reasons, a method has been developed in which a signal of a minute amplitude is superimposed on the injection current of an LD to effect a digital frequency modulation or frequency shift keying (FSK), and the demodulation is achieved by utilizing the wavelength discrimination characteristic of the optical filter. In this connection, reference should be made to M. J. Chawki et al. "1.5 Gbit/s FSK Transmission System Using Two Electrode DFB Laser As A Tunable FSK Discriminator/Photodetector", Electron. Lett. Vol. 26 No. 15, 1990.
The DFB filter as shown in FIG. 1, however, has polarization dependency, which results from the fact that a tuned wavelength (i.e., the selected wavelength of light that is transmitted through the DFB filter) for light having an electric field component parallel to the layer surface of the device (TE mode) is different from a tuned wavelength for light having an electric field component perpendicular to the layer surface of the device (TM mode). The difference is caused by the following phenomenon. Since effective indices of the waveguide for TE and TM modes are different, the Bragg conditions of the diffraction grating, which are EQU .lambda.=2n .LAMBDA./m
(.lambda.: wavelength of light, n: effective index, .LAMBDA.: pitch of diffraction grating, m: integer or order of diffraction grating),
deviate from each other between the TE and TM modes. In the filter shown in FIG. 1, there are arranged a waveguide 341, a grating 342 formed in the waveguide 341, three electrodes 343, 344 and 345 separated from each other along the light propagation direction, an active layer 346 and anti-reflection films 347 and 348 deposited on opposite end surfaces of the filter. The electrodes 343 and 345 at opposite end portions (active regions) serve to cause a signal gain, and they change the refractive index of the waveguide 341 by changing carrier densities therein and change the wavelength reflected by the grating 342 in a distributed manner. The electrode 344 at a central portion (a phase adjusting region) changes the carrier density distribution therein to control the refractive index, and thus changes the phase of light propagated through the waveguide 341 to achieve the wavelength tuning in a wider range.
In general, the index n for TM mode is smaller than the index n for TE mode, and hence the tuned wavelength for TM mode is shifted toward a shorter wavelength side, relative to the tuned wavelength for TE mode. Therefore, if the tuned wavelength of a DFB filter is adjusted so that its gain is maximum, for example, for TE mode, the transmission intensity of the filter changes with time because the TE mode component varies when the polarization plane of signal light is rotated during transmission in the optical fiber. As a result, the received intensity varies with time, and a reduction of receiving sensitivity and an increase in error rate result. In the worst case, signal receiving is barely achieved if all the signal is coupled to the DFB filter in TM mode.
When FSK transmission, in which a high-density wavelength or frequency multiplicity is possible, is conducted, wavelength tracking for attaining stable reception is difficult since demultiplexing is conventionally performed by a single filter at the receiver side. The reason therefor is that there are mark and space frequencies respectively corresponding to "0" and "1" of a signal in the case of FSK transmission A separation or selection of a signal at the space frequency is difficult when the tuned wavelength of the filter is stabilized at the mark frequency, for example.